If another driver hits your vehicle, you might expect that person’s insurance to pay for the damage. But what if he or she doesn’t have coverage?

Enter uninsured and underinsured motorist insurance. These coverage options can spare you from shelling out your own money for crashes you didn’t cause. In some states they’re required. In others they’re optional. But they generally don’t cost much to add to your auto policy.

Uninsured motorist coverage at a glance

When you would use it: If you’re in a crash caused by an uninsured driver, or a hit-and-run crash.

What it pays for: Injuries you or your passengers suffer.

Also good to know: There’s a separate option that pays for damage caused to your car or other property, called uninsured motorist property damage coverage, or UMPD. If you make a claim on UMPD, you might have to pay a deductible, depending on your state. Some states require UMBI coverage only, while UMPD is optional. In other places, both are either optional or mandatory.

Alternatives:

Health insurance will pay for medical treatment after a car wreck.

Collision coverage will pay for damage to your car if an uninsured driver hits it. Your claim check, though, would be reduced by the amount of your collision deductible.

Underinsured motorist coverage at a glance

When you would use it: If you’re hit by a driver who has some insurance, but not enough to cover your medical costs, this coverage can make up the difference.

What it pays for: Injuries you or your passengers suffer.

Also good to know: If you also select underinsured motorist property damage coverage, or UIMPD, your policy pays for vehicle repairs that the at-fault driver’s insurance won’t fully cover. A few states mandate all of these coverages — UMBI/UIMBI and UMPD/UIMPD.

Alternatives:

Health insurance will pay for medical treatment after a car wreck.

Collision coverage will pay for damage to your car if you’re hit by a driver who doesn’t have enough auto insurance. However, your claim check would be reduced by the amount of your collision deductible.

The cost

NerdWallet analyzed car insurance rates in three states for 30-year-old motorists with clean driving records.

We looked at costs with and without UMBI/UIMBI and UMPD/UIMPD.

Of the states we analyzed, California had the highest rate increase after adding these coverage options, roughly $9 per month. Illinois was the cheapest, with an average increase of around $3 a month.

Of the states we analyzed, California had the highest rate increase after adding these coverage options, roughly $9 per month.

These are relatively minor costs compared with the steep medical or repair bills you could face after a crash with an uninsured driver. Even if they’re not required where you live, uninsured and underinsured motorist coverage are worth considering.

Methodology

NerdWallet researched car insurance rates from the largest insurers for 30-year-old men and women across 10 ZIP codes in California, Illinois and Pennsylvania.

We looked at rates with:

$100,000 bodily injury liability per person.

$300,000 bodily injury liability per accident.

$50,000 property damage liability per accident.

Collision and comprehensive coverage each with a $1,000 deductible.

We averaged the three lowest rates for each state. We then added UMBI/UIMBI and UMPD/UIMPD with the same limits as liability coverage and looked at rates again (except in California, which offers a $3,500 collision deductible waiver instead of UMPD). After averaging the three lowest rates, we compared the difference in price between the policies without uninsured/underinsured coverages, and with them. We used a 2013 Toyota Camry in all cases.

These are sample rates generated through Quadrant Information Services. Your own rates will be different.

Understanding auto insurance

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